Constantly rising prices for heating oil and natural gas in households and the environmental impact lead to a rising interest in renewable raw materials. Wood pellets are small cylindrical compacts from natural wood, primarily from sawdust and wood shavings. They are used as a versatile biofuel with a lower emission of the climate-relevant carbon dioxide. These pellets can be burned using a variety of technologies, such as home heating stoves or institutional boiler systems. It is therefore important to look on the overall risks involved in handling these pellets. Mechanical forces during transportation cause fractures and breakage of pellets, resulting in fines and dust. A high content of fine particles can block the conveyor systems. In addition to the fine content, an increased water content due to storage under conditions with high humidity can also lead to a reduction in quality, e.g. by decreasing the flowability of the wood pellets and thus their ability to be conveyed.
The aim of this research is to investigate the formation of fines and dust as well as the influence on the flow behavior of wood pellets during the entire storage and drain process. In addition, the influence of the pellets’ water content on their mechanical stability will be quantified. Therefore, the relevant material parameters (e.g. Young’s modulus, coefficient of restitution) will be determined after storage under defined temperature and humidity conditions. The obtained correlations will then be used to simulate the storage and transport as well as local segregation, water migration into the bulk and the diffusive water transport. The findings from the numerical investigations will be validated by experimental investigations with a lab-scale silo.
Various methods are used to investigate the material properties of wood pellets with different water content. A defined water content is realized using a climate chamber (ICH110 by Memmert GmbH & Co. KG). Subsequently, mechanical properties, such as pellet density (Micromeritics 1305), coefficient of restitution and modulus of elasticity (Texture Analyzer TA.XTPlus), which are important for numerical models (DEM), will be determined. In addition to the studies of the mechanical properties, it is also necessary to understand the diffuse transport of water within a pellet and the pellet bulk. Sorption isotherms provide information about the diffuse transport of water vapor in a porous material.
Figure 1 shows sigmoid shaped adsorption curves, which can be classified as type II according to the classification of van der Waals sorption isotherms. It can be seen that the equilibrium water content in wood pellets depends more on the humidity than on the temperature. Further experiments are carried out to investigate the capillary water transport and the water distribution in the pellets and the pellet bulk. In the end, a predictive model will be obtained, which describes the mass transfer in the silo in dependence on the surrounding conditions. Furthermore, the drain process of a pellet bulk from the storage by means of different conveyor systems, e.g. by screw conveyors, will be investigated in detail. Screw conveyors are placed in different positions into the bulk. The torque as a function of the filling configuration is used to quantify the conveying properties. To estimate the delivery rate, the mass flow of the pellets and the
actual utilization of the maximum possible delivery volume of pellets can be determined. Since measuring access in submerged systems is generally more difficult, DEM (Discrete Element Method) simulations will be used to gain an understanding of the relationships between the various system parameters.
The German Federation of Industrial Research Associations (AiF). AiF Project 20565 N / 2
Department of Energy Plant Technology, Ruhr-University Bochum (Prof. V. Scherer)